This makes `versionadded` and `versionchanged` directives show up in
`cmake --help-*` output instead of disappearing (and potentially making
empty sections).
Fixes: #22808
This enables cross-reference syntax for CMake generator expressions:
:genex:`SOME_GENEX`
:genex:`$<SOME_GENEX>`
:genex:`$<SOME_GENEX:...>`
and definition of CMake generator expressions via a directive:
.. genex:: SOME_GENEX
.. genex:: $<SOME_GENEX>
.. genex:: $<SOME_GENEX:...>
It also adds generator expressions defined by the directive and by
`Help/genex/SOME_GENEX.rst` documents to the index.
This was accidentally left out of commit 8acf46caf1 (Utilities/Sphinx:
Add role and directive for 'envvar' in CMake domain, 2018-04-19,
v3.12.0-rc1~200^2~1).
Remove this content from the `cmake-developer(7)` manual because it
is relevant only to developers working on CMake itself. Move it to
a guide in the developer documentation.
